1. Field of the Invention
The present invention relates to locking devices which are used to provide torque to prevent a threaded fastener (bolt and nut, cap screw, or the like) from rotating and, more particularly, to a locking device which, in addition to providing torque, does not bear any preload stress from the fastener head.
2. Prior Art
In many applications, bolts and nuts or cap screws are used to join components. A significant number of these applications require that these threaded fasteners be tightened to a very high torque. This torque creates a large amount of preload stress between the fastener head (or nut) and the component. Even with these highly torqued fasteners, a common problem is that, under certain conditions, particularly when the components vibrate, the fasteners tend to work loose. This problem has been addressed in the prior art by the use of several devices which are designed to prevent an individual bolt or nut from rotating, including various types of lock washers, lock nuts, castle nuts with cotter pins, etc. Each of these devices has its own drawbacks, however. Lock washers fit between the bolt head (or nut) and the component and, therefore, must bear the preload stress that is incurred by tightening the bolt. Additionally, lock washers provide only a limited amount of torque to prevent rotation of the bolt. Lock nuts, while typically not bearing any preload stress, also provide a limited amount of torque to the bolt. Castle nuts with cotter pins, like several other types of locking devices, require a particular orientation of the bolt (or nut) and, therefore, do not allow the bolt to be tightened as desired.
Another type of locking device, called a locking plate and shown in FIG. 1, has been developed for use with two or more adjacent bolts. In FIG. 1, a locking plate L is shown used with fasteners (bolts), having fastener heads (bolt heads) f1 and f2, that hold a first component C1 to a second component C2. As illustrated, the locking plate L has a flat base portion having holes (not shown), each of which is positioned between a respective bolt head (f1 or f2) and the component C1, much like a washer, and the tabs b which can be bent to wrap around the sides of the respective bolt head f1 or f2 to secure the bolt head in a particular orientation with respect to the base portion. Thus, because the base portion is constrained from rotating by at least two bolts, the bolt heads (f1 and f2) also cannot rotate.
The lockplate L has several drawbacks. First, the tabs b often are not aligned properly with the bolt head when the bolt head is tightened to the desired torque and, therefore, the tabs b provide reduced holding power. Second, because the lockplate L is positioned between the bolt heads f1 and f2 and the component C1, it bears the preload stresses created by tightening the bolts. While these stresses are not too large for a hardened bolt, they are too large for the locking plate L which must be made from a softer material in order to allow the tabs to be bent around the bolt head. This softer material yields under high preload stresses. Moreover, hardened material cannot be used for the lock plate because, while extreme force might be applied to bend tabs made from hardened material, such force could cause the hardened material to crack. Additionally, while providing additional material under the bolt head would increase the load capacity of the soft lockplate, this solution is difficult to implement because the diameter of the holes in the lockplate must include sufficient tolerance to allow for variations in fastener diameter as well as variations in the distance between fasteners. Further, the use of larger fastener heads is also not a suitable solution because many applications require a particular size fastener head, either because of space, material, or financial constraints.